A frequency synthesizer comprises an electronic system which generates at its output a higher frequency signal(s) from the lower frequency signal received from a single fixed time base or master oscillator. A common way to implement a frequency synthesizer is with a phase-locked loop (PLL).
A PLL is a feedback control system that includes an error detector (comprising a phase frequency detector coupled to a charge pump) which compares the phases of two input signals (reference frequency signal and frequency divided higher frequency output signal) to produce an error signal that is proportional to the difference between their phases. The error signal is then low pass filtered and used to drive a voltage-controlled oscillator (VCO) which creates the higher output frequency. The output frequency is fed back through a frequency divider to the input of the phase frequency detector, producing a negative feedback loop. If the output frequency drifts, the phase error signal will increase, driving the frequency in the opposite direction so as to reduce the frequency error. Thus, the output is locked to the frequency at the other (reference) input of the error detector. This reference input is usually derived from a crystal oscillator, which is stable in frequency.
One application for frequency synthesizers is for enabling flexible and cost-effective implementation of frequency modulated continuous wave (FMCW) radar systems. For example, automotive radar systems use frequency synthesizers to generate a continuous wave (CW) of constant frequency or time-varying frequency. Since the driver's safety is critical in automotive applications, it is important to continually monitor the performance of the frequency synthesizer with respect to the phase noise in the frequency synthesizer output continuously. Higher phase noise during on field operation relative to a certain acceptable noise level expected during the design of the radar apparatus can cause the radar apparatus to potentially fail to detect some surrounding obstacles. False detection of obstacles where there is actually none is also likely in the presence of phase spurs in the synthesizer output. Hence high phase noise or spurs may render the radar measurements unreliable.